1. Field of the Invention
The present invention relates to a heat-dissipating fan. In particular, the present invention relates to an axial-flow heat-dissipating fan.
2. Description of Related Art
FIG. 1 of the drawings illustrates an impeller 10 for an axial-flow heat-dissipating fan. The impeller 10 is mounted in a casing 20 and includes a hub 101 and a plurality of blades 102. Each blade 102 is mounted on an outer periphery of the hub 101 in an inclined angle. The blades 102 drive air to flow in an axial direction. Due to the limitation of release of the mold forming the impeller 10, two blades 102 adjacent to each other cannot overlap with each other as viewed from the longitudinal direction parallel to the rotational axis of the impeller 10. The total amount of air driven by the impeller 10 is in proportion to the number or total air-driving area of the blades 102. In other words, the total amount of air driven by the impeller 10 can be increased only by overcoming the release limitation of the mold.
A complex impeller consisting of two hub parts has been disclosed in, e.g., U.S. Pat. Nos. 6,318,964 and 6,572,336. As illustrated in FIGS. 2 and 3, such a complex impeller 3 comprises a shaft 30, a complex hub 31, and a plurality of blades 32. The complex hub 31 includes an upper hub 31a and a lower hub 31b. A plurality of upper blades 32a are formed on an outer periphery of the upper hub 31a, and a plurality of lower blades 32b are formed on an outer periphery of the lower hub 31b, with the upper blades 32a and the lower blades 32b together forming the blades 32. Each blade 32 overlaps with an adjacent blade 32 as viewed from a longitudinal direction parallel to the shaft 30.
As illustrated in FIG. 3, after assembly, the leading edge 321 of each blade 32 is coincident with the trailing edge 322 of an adjacent blade 32. By such an arrangement, the number of the blades 32 and the total air-driving area of the blades 32 are increased. However, the blades 3 are not disposed properly such that the blowing noise is high.
The impeller 3 can be further assembled with a casing (not shown) to form a heat-dissipating fan. In operation, the assembled blades 32 drive air to exit the casing via an air outlet of the casing. However, the trailing edge 322 of each blade 32 and the supporting ribs in the air outlet of the casing are not well designed such that the outgoing airflow impacts the supporting ribs in the wrong direction, generating unacceptable noise in the area adjacent to the supporting ribs.